1. Field of the Invention
The present invention relates to an elastic body made of elastic material such as addition reaction type silicone rubber, and more particularly, it relates to an elastic body used for forming rollers such as fixing rollers, feed rollers and the like or blades such as cleaning blades, developing blades and the like, which must have the good mold-releasing ability and are used with an image forming system such as an electrophotographic copying machine, electrophotographic printer and the like.
Further, the present invention relates to an elastic roller using such elastic body, and a fixing device having such elastic roller.
2. Related Background Art
Generally, feed rollers having a pair of elastic rollers, for feeding a sheet-like transfer material or a recording sheet such as a plain paper sheet, OHP film and the like (which is used with an image forming system such as an electrophotographic copying machine, electrophotographic printer and the like) along a predetermined path are required to have the good feeding ability, mold-releasing ability and durability.
The blade such as a cleaning blade for removing toner and/or dirt remaining on a photosensitive member used with the image forming system, a developing blade for forming a thin toner layer on a developing sleeve and the like is functionally disposed in sliding contact with the toner and thus is required to have the good mold-releasing ability and high wear resistance and therefore high durability.
The feed rollers used with the image forming system and having a pair of elastic rollers for feeding a recording sheet on which a non-fixed toner image was formed must have the good mold-releasing ability, high wear resistance and high durability, since they must pressurize the toner image on the recording sheet and, if necessary, heat the toner image to fix onto the recording sheet, and feed the recording sheet along a predetermined path while preventing the offset phenomenon.
Heretofore, a fixing roller is comprised of a metallic core, and a coating layer disposed around the core and made of good mold-releasing material such as fluororesin or silicone rubber. In the image forming system, the fixing roller having the silicone rubber coating layer which is inferior to the fixing roller having the fluororesin coating layer in the mold-releasing ability and durability but superior to the latter in the provision of the good image quality tends to be used in view of the importance of the image quality, and thus the improvement of the mold-releasing ability and durability of the silicone rubber material has been investigated and developed.
In color electrophotography, in order to reproduce an image color as close to an original image color as possible, since a plurality of color toners are heat-fused and mixed, it is necessary to use the toners having a low softening point. When the toner having the low softening point is used, if the toner image is thermally fixed by a fixing roller having the fluororesin (for example, Teflon) coating layer which has been used with a general copying machine for obtaining a black and white image, the offset phenomenon that the toner is adhered to the fixing roller will be generated noticeably, thus making the formation of the good fixed image difficult extremely. To avoid such offset phenomenon, it has been tried to always apply silicone oil (as toner separating agent) to a surface of a fixing roller having an outer coating layer made of silicone rubber material with the good mold-releasing ability and special grade.
However, the mold-releasing ability of the existing silicone rubber materials is still insufficient, and, therefore, the improvement of the mold-releasing ability and durability of the silicone rubber material has been investigated and developed.
For example, the Japanese Patent Appln. Laid-Open No. 61-144675 teaches the fact that, in the addition reaction type silicone rubber materials, hardened matter comprised of straight-chain dimethyl polysiloxane terminated with vinyl groups having molecular weight of 20000-200000 has the good mold-releasing ability.
According to the above Japanese gazette, it is said that as the molecular weight of the straight-chain dimethyl polysiloxane terminated with vinyl groups is greater, the mold-releasing ability get higher. However, if the molecular weight exceeds 200000, the physical strength of the matter will be doubtful, and thus such matter cannot be used practically.
Now, the relation between the molecular weight and the viscosity of the straight-chain dimethyl polysiloxane will be briefly described. In general, when the molecular weight of the dimethyl polysiloxane is increased, the viscosity is also increased; in case of the straight-chain dimethyl polysiloxane, the molecular weight of 200000 corresponds to the viscosity of about 60000 poise.
On the other hand, in the conventional coating layers of the roller, when the rollers having the mold-releasing agent applied thereto are used as the fixing rollers under the heated condition at high temperature, the mold-releasing agent enters into the outer layers of the rollers, thus deteriorating the silicone rubber constituting the outer layers. Such deterioration is particularly noticeable in the outer layer of the higher temperature roller.
To avoid such deterioration of the outer layers, the Japanese Patent Publication No. 54-41330 proposes providing an intermediate layer as an oil barrier layer made of fluororubber and the like between the silicone rubber layers as the outer layer of each roller. Further, as disclosed in the Japanese Patent Publication Nos. 54-41332 and 57-46068, it has been proposed that an underlayer was formed by mixing the base material with non-swelling material (i.e., material hard to be swollen). However, when the deterioration of the silicone rubber layer is prevented with the above-mentioned method, it is necessary to select materials having excellent mold-releasing ability as the outer layer material of the roller, with the result that at least two layers are required to constitute the outer layer of the roller.
Alternatively, Japanese Patent Publication No. 54-26373 teaches the fact that silicone rubber constituting the rubber layer used as the outer layer of the roller and silicone oil applied thereto are selected so as not to cause the swelling of the rubber layer, and the selected silicone rubber and silicone oil are combined. However, according to this Japanese gazette, it is said that the combination of addition silicone rubber and phenylmethyl silicone oil is not preferable because of the occurrence of the swelling.
Further, the Japanese Patent Appln. Laid-Open No. 3-33786 discloses a heat fixing roller comprising a metallic core member, an elastic layer made of silicone rubber, an oil barrier layer made of fluororubber, and a mold-releasing layer made of addition silicone rubber, where such layers are arranged around the core. With the arrangement of this heat fixing roller, the mold-releasing agent is prevented from penetrating into the elastic rubber layer by the oil barrier fluororubber layer, and the toner separating ability, lipophilic ability to the silicone oil, surface smoothness and wear resistance are provided on the roller surface by the mold-releasing agent or surface lubricant.
However, the mold-releasing layer formed on the surface of the heat fixing roller disclosed in this Japanese Patent Appln. Laid-Open No. 3-33786 is formed by curing the mixture comprised of 100 weight part of methylvinyl polysiloxane ((a) component) having at least two vinyl groups per molecule and a polymerization degree of 5000-12000, and 1-100 weight part of methylvinyl polysiloxane ((b) component) containing triorganosilhemioxane unit (M unit) and silicate unit (Q unit) in a specific rate. More particularly, in an embodiment, since it is said-that the heat fixing roller having the mold-releasing layer formed by combining 100 weight part of the (a) component and 30 weight part of the (b) component containing the M unit and Q unit has the durable offset life for 30000 sheets, it is considered that the methylvinyl polysiloxane constituting the (b) component is polymer having resin segment comprising the M unit and Q unit as a main component. Accordingly, the methylvinyl polysiloxane constituting the (b) component in this embodiment has a resin segment comprising the M unit and Q unit as the main component, and a soft segment comprising continuous diorganosiloxane unit (D unit) has only a short, hard resin skeletal structure wherein the number of repeating units is several tenths at the most. The mold-releasing layer formed by curing the mixture of the (b) component methylvinyl polysiloxane and the (a) component methylvinyl polysiloxane limits the service life of the roller, since the mold-releasing ability of the (b) component is insufficient. Further, if the mixing rate of the (b) component is increased, the elastic body obtained by heat curing the mixture will become harder and fragile.